The present invention relates generally to switching circuits and, more particularly, to a current sensing automatic switching network for switching between transceivers on computer networking systems.
It is common practice in the computer utilization art to interconnect computers and their peripheral devices into a local area network to allow the exchange of information between computers, share input and output devices and to access the network from remote points. The Ethernet system which interconnects two or more computers or workstations into a network is well established in the art and has become a standard network with well defined parameters, allowing devices from dissimilar manufacturers to be joined together. Communication over the network between each computer, input device or output device thereon, is performed by a component within each unit commonly referred to as a transceiver. The Ethernet is a narrow band network system that has no central control. Consequently, two or more transceivers may communicate over the network only sequentially. When a transceiver wishes to transmit over the network, it must first "listen" to determine that no other transceiver is transmitting. When any one transceiver is transmitting over the network all other transceivers on the network must wait their turn to avoid overlapping messages which result in unintelligible communications. This sequential operation is not problematic with simple networks but becomes a substantial limitation as the network increases in complexity. This problem is manifest as communication delays between devices.
The problem of communication delay has been addressed in several ways. Networks may be subdivided into smaller sub-networks which are then joined together into a large network with switching or filter devices such as bridges or gateways. Bridges connect two sub-networks so that only communication addressed to devices on the other network are passed over. This allows the communications traffic to be localized and filters out traffic unnecessary for either sub-network's activities. A gateway typically connects several Ethernets or other networks. The gateway receives and stores a message, looks up the destination address attached to the message in a table and then routes the message over the appropriate network to the proper node. These functions take time and as the communications traffic increases, the delays inherent in the system become significant.
One method to minimize the time delays in switching circuits is to limit the storage time while the circuit determines the proper destination. This is accomplished by reading only the address associated with the communication and routing the message immediately, thus achieving a routing decision while the first transceiver is still transmitting. This on-the-fly routing capability allows the message to be switched to the appropriate sub-network and received at the proper node while the sending transceiver is still transmitting. The circuitry for this fast logic switch is both complex and expensive, utilizing numerous expensive components.
The cost of conventional bridges, gateways and on-the-fly switches is difficult to reduce since their standard components are presently produced in large quantities at great cost and have not reflected the conventional economies of mass production. Creation of custom components using application specific integrated circuit technology is difficult to integrate with present technology and would not likely yield any significant cost reduction in switching circuitry.
There are numerous commercially available plug-in circuit boards or adapter cards which serve to interface various devices, such as desktop personal computers, to external networks. These adapter cards have been standardized in the art and are generally referred to as an Attachment Unit Interface (AUI port). These adapters all require some sort of switching device, either manual or automatic, to select either an internal transceiver connected to one external network or an external transceiver on an alternate external network. Typically, commercially available prior art Ethernet adaptor cards require a user to manually set a physical switch to select the external transceiver and alternate network when the cable for the alternate network is connected to or plugged into the device AUI port. If a manual switch should be inadvertently left in the incorrect position, both internal and external transceivers are accessed simultaneously, resulting in intermittent operation of both and subsequent data loss. Further, most prior art installations include card mounted components access to which is gained only by opening the computer or other device case thus greatly increasing the time required to connect an alternate network to the computer.